This section provides background information to facilitate a better understanding of the various aspects of the invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Submersible pumping systems have been employed in the pumping of oil and water from wells for many years. Commonly, a submersible pumping system comprises an electric motor, a motor protector and a pump suspended colinearly in a well by tubing or cable. The pump is generally a centrifugal pump which is coupled to the motor. The motor rotates a power transmission shaft that concurrently operates the pump. The motor and motor protector are filled with oil to aid in heat dissipation, to maintain proper internal lubrication of the motor, and to separate the internal components of the motor from surrounding wellbore fluids.
Because these pumping systems are generally disposed within a narrow well, the motor, motor protector, and pump are generally long and cylindrically shaped. The motors vary in horsepower depending on the application. Accordingly, the motors of submersible pumping systems can be quite long leading to particular difficulties not encountered in other electric motor applications.
The motors of submersible pumping systems typically comprise a stator secured within a tubular housing and a rotor secured to a power transmission shaft that rotates within the stator. The rotor typically is made up of a number of rotor sections, the number of rotor sections depending upon the length and power rating of the motor. Generally, each rotor section comprises laminated steel plates or disks secured by copper rods. The rotor sections are spaced apart from each other, and a rotor bearing assembly is located between each rotor section. Each rotor section is connected to the shaft so that all of the rotor sections rotate as the shaft rotates.
Each rotor bearing assembly within a rotor section acts to support the shaft and to maintain it in proper axial alignment. A rotor bearing assembly comprises a sleeve connected to the shaft so that the sleeve and shaft rotated together and a journal (e.g., bearing, bushing) disposed coaxially around the sleeve. The sleeve and journal are rotatively coupled to one another. The journal may be configured to frictionally engage the inner wall of the stator (e.g., housing) to prevent the journal from rotating and to maintain proper alignment of the shaft. Thus, a portion of the rotor bearing assembly is rigidly coupled to the shaft but not to the stator.
Due to the high operating temperatures within the well, thermal expansion tends to cause the shaft, rotor, and stator to grow axially. Generally, the rotor and shaft tend to grow axially downward during high temperature operation. The stator also tends to grow axially downward, however, to a lesser extent than the rotor and the shaft. Due to these thermal expansion effects, the motor is constructed so that each rotor bearing assembly attached to the motor shaft within a rotor section offers a limited amount of axial mobility. Thus, because each rotor bearing assembly is coupled to the motor shaft, the shaft retains the same limited amount of axial mobility. In some pumps, axial mobility is limited by thrust washers adjacent to each rotor bearing assembly.
Angular misalignment of the shaft within the motor can occur because the rotor, shaft, and stator are subject to these dimensional changes due to thermal expansion and because of imbalances in the rotating assembly. Misalignment of the shaft during operation opposes the centering, or aligning force of the bearing assembly and causes vibrations within the motor. Excess vibration can lead to premature motor or component failure.
Ideally, the journal remains stationary while the sleeve, rotor, and shaft are rotating. Rotor bearing assemblies have been used in which the peripheral surface of the journal frictionally engages the inner surface of the stator through metal-to-metal contact, such as via a metallic washer. Such metal-to-metal frictional fit rotor bearing assemblies have a tendency to become loose and then to rotate with the shaft. Rotation of the journal tends to gouge and deface the inner surface of the stator. Once the journal begins to rotate with the shaft, the centering force of the rotor bearing assembly is diminished leading to increasing angular misalignment, vibration, and motor failure. This type of construction is also unsatisfactory because due to thermal expansion of the bearing assembly during motor operation, the journal may tightly engage the stator wall which can cause angular misalignment of the shaft and thus excessive thrust loads onto the thrust bearing surfaces adjacent to the rotor bearing assembly.
Some electric submersible pumps utilize ceramic carbide (e.g., tungsten carbide, silicon carbide, aluminum nitrite, boron carbide, cobalt) bearings (e.g., sleeve and/or bushing) to resist the abrasive action of sand or other hard particles in the well fluid and to function with very low viscosity lubrication. A major challenge with ceramic carbide devices is securing the mating bearing components in a manner that does not create serious stress raisers that make the ceramic carbide susceptible to cracking. Cracking may be caused by shock loads encountered during shipping, handling or installation. Cracking may also be caused by thermal expansion stresses due to running in a poor lubricant that insufficiently lubricates or cools the bearing, such as low viscosity fluid or in a well fluid with a high gas content. Cracking may also be caused by axial or transverse shocks during operation, particularly as the pump shaft constantly moves upward and downward during gas slugging. A catastrophic pump failure may occur, if even one of the cracked bearings (e.g., sleeves) in the rotor (e.g., impeller) stack actually breaks apart.